Switching amplifiers are used in various electronic components such as audio amplifiers. A switching amplifier consists of a network of switching elements that produces a square wave output and is connected to a load through a power stage. The switching amplifier typically uses pulse width modulation (PWM). The MOSFETs are switched either on or off, rather than operated in linear mode, to convert an input signal to a sequence of pulses (a PWM signal) whose averaged value is directly proportional to the amplitude of the input signal at that time. The frequency of the pulses is typically ten or more times the highest frequency of interest of the input signal. The switching amplifier is used in audio equipment, for example, when reproducing analog signals stored on a compact disc in a digital format.
The PWM switching period is typically fixed. The ratio of the time that a PWM signal is high to the switching period is called the duty ratio. A digital PWM signal is defined by the switching period and the duty ratios. During digital processing, the PWM duty ratio is quantized, resulting in a signal-to-noise ratio that is reduced from that of the original signal. The quantized PWM signal introduces audible distortion and raises the noise level of the audible frequency band. Accordingly, noise shaping is used on the PWM signal to manipulate the signal to reduce the noise level and force the noise out of the audible range. Noise shaping uses a noise shaping loop within the digital processor.
In audio equipment using the switching amplifier, at peaks of audio and high volume settings, the desired output voltage exceeds the capability of the power supply and the power stage. During these times, the largest output power can be obtained by stopping switching of the MOSFETs and allowing the entire power supply across the load. However, large input signals can cause the noise shaping loop to become unstable.
One approach to increase the stability is to artificially limit the amplitude of the input signal to a lower value, and thus the duty ratio of the PWM signal, before the noise shaping loop goes unstable. However, this reduces the peak power out of the amplifier due to both the reduction in duty ratio range and the relative inefficiency of the power stage when operated with a very high or low duty ratio. Additionally, artifacts in the audio signal may be introduced by the transition from the switching mode to the saturation mode (and back) of the switching amplifier. Further, if a lookup table is used to provide the amplitude limitation, a large amount of memory (which is relatively expensive) is used, and the power consumption and size of the device is increased.
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